With the emerging demands for clean energy and an economy with net-zero greenhouse gas emissions, electrocatalysis areas have attracted tremendous interest in recent years. The electrochemical devices that use electrocatalysis, such as fuel cells, electrolyzers, and flow batteries, consist of hierarchical structures, requiring comprehension and rational designs across scales from millimeter and micrometer all the way down to atomic scale. In past decades, electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been extensively utilized for imaging different scales of these devices in both two and three dimensions. However, electron-based techniques for high-resolution imaging require uninterrupted maintenance of a high-vacuum environment, leading to difficulties of sample preparation and lack of integrated observation without intrusion/disassembly. To overcome these disadvantages, more and more efforts have been dedicated to the development of X-ray imaging techniques recently, specifically absorption-based two-dimensional (2D) transmission X-ray microscopy and three-dimensional (3D) X-ray tomography, due to much better transmission behaviors of X-rays than electrons. X-ray tomography imaging mostly focuses on answering questions related to morphology and morphological changes at the microscale or near 1 μm resolution and nanoscale of 30 nm resolution. The method is nondestructive and it allows for the visualization of operando electrochemical devices, such as fuel cells, electrolyzers, and redox flow batteries. Operando X-ray microscopic tomography typically focuses on catalyst layers and morphology changes during degradation, as well as mass transport. Nanoscale tomography still predominantly is used for ex situ studies, as multiple challenges exist for operando studies implementation, including X-ray beam damage, sample holder design, and beamline availability. Both microscale and nanoscale tomography beamlines now couple various spectroscopic techniques, enabling electrocatalysis studies for both morphology and chemical transformations. This viewpoint highlights the recent advances in X-ray tomography for electrocatalysis, compares it to other tomographic techniques, and outlines key complementary techniques that can provide additional information during imaging. Lastly, it provides a perspective of what to anticipate in coming years regarding the method use for electrocatalysis studies.

Methanol may play a major role in a hydrogen economy by serving as one of the highest energy density compounds available; however, the precise reaction pathways for methanol oxidation catalysts have yet to be fully elucidated. Herein, a combination of packed-bed reactor studies and high-vacuum surface science techniques was used to elucidate the reaction mechanism of methanol oxidation over a Pt/TiO2 catalyst. The reactor studies highlight that methyl formate is produced under mild reaction conditions, and full combustion to CO2 is achieved at elevated catalyst temperatures. The surface science experiments show that the production of CO2 proceeds through a surface-bound formate intermediate via multiple proton-coupled electron-transfer steps. Importantly, we also find that the water produced upon initial methanol adsorption plays a key role in unlocking the oxidative chemistry of this Pt-based material. These results provide valuable insight into potential modifications that could preferentially direct catalyst activity toward partial or full oxidation, thereby unlocking methods for producing valuable commodity chemicals.

Artificial metalloenzymes, which are designed rationally as hybrids of proteins and catalytically active transition-metal complexes, have become a promising approach for catalyzing unprecedented reactions for natural enzymes. In this study, we described the design and synthesis of an artificial metalloenzyme, a cycloisomerase that utilizes Au(I) incorporated into an apo-ferritin cage (Fr–Au) to efficiently catalyze the cycloisomerization of alkynoic acids, with a conversion of 83% and a turnover frequency of 20.6 × 103·h–1 in aqueous solution under mild conditions. The remarkable catalytic activity indicates that the nano-confinement of the Au(I) active site within the ferritin cage enhances its catalytic properties by stabilizing and solubilizing it. The less protected Au atom in the cysteine bridged dinuclear Au(I) active center was identified as critical for the Fr–Au cycloisomerases to catalyze this reaction. In addition, we provide insight into the catalytic mechanism through quantum chemical (QC) calculations, which reveal an energy barrier of 32.29 kJ/mol.

Fe-modified BiVO4 represents a promising anode material for the photoelectrochemical (PEC) oxygen evolution reaction in neural electrolytes, the bottleneck reaction in PEC water splitting. To reveal the catalytic role of Fe in this composite catalytic system, here we utilize combined theoretical and experimental techniques to identify the location and structure of FeOx phases and optimize the catalytic performance. By using the machine-learning interface search method, we screen out a coherent ε-FeOOH1.5(011)/BiVO4(001) interface from thousands of likely interface candidates. The interface has a low formation energy (0.74 J/m2), a narrow band structure (∼1.6 eV), and desirable catalytic activity (reaction barrier ∼ 0.64 eV) when the ε-FeOOH1.5 overlayer is two atomic layers thick. Guided by the theoretical findings, our orthogonal PEC experiments are performed to identify the optimal synthetic conditions. The best PEC activity reaches 5.4 mA/cm2 (1.23 V vs reversible hydrogen electrode) when using FeSO4 as the precursor with the chemical bath method at 40 °C for 4 h, which is ∼0.9 mV/ cm2 higher compared to the previous experiment. By analyzing transmission electron microscopy (TEM) pictures and performing TEM simulations, we confirm that the grass-like FeOx structures grown on BiVO4 are ε-FeOOH crystals as predicted by theory.

Methane pyrolysis is a unique approach toward generating hydrogen and valuable carbon products, with the added advantage of low to near-zero CO2 emissions. Currently, the most popular method for hydrogen production is steam methane reforming, which generates more than 10 kg of CO2 for every 1 kg of hydrogen. By comparison, methane pyrolysis produces hydrogen and solid carbon with no COx biproduct. Methane pyrolysis on a conventional solid catalyst exhibits low activation energy, but the carbon coproduct cannot be separated and rapidly poisons the surface (coking). On the other hand, molten liquid metal catalysts have been shown to have the advantage of separatable carbon, but their high activation energy limits the rate of the reaction and potential for economic industrialization. In this work, methane pyrolysis was shown using a multiphase molten metal reactor where both liquid and solid metal alloy catalysts were in equilibrium. Catalytic measurements using a Sn–Ni melt showed that operating the reactor in the two-phase region of the Sn–Ni phase diagram decreased the apparent activation energy from 355 kJ/mol in the liquid-only melt to 158 kJ/mol, all while maintaining the ability to separate and recover graphitic carbon.

Water addition on an unsymmetrical internal alkyne has significance in organic chemistry and has stimulated challenging approaches to access a highly regioselective process. Hydration or formal hydration is presented according to the type of internal alkyne. Methods for electron-rich and -poor diarylalkynes followed by those adapted to arylalkylalkynes are presented. Regioselective hydration reactions of unsymmetrical propargylic and homopropargylic derivatives are then discussed, followed by regioselective aspects of the hydration of internal alkynes directed by carbonyl groups. The following section examines the hydration alkynes linked to CF3, esters, ketones, and heteroatoms as haloalkynes, alkynylphosphonates, ynamides, and others. At the end of this section, we report some recent examples of hydration reactions where the alkyne triple bond is connected to an electron-withdrawing group as an ester or a ketone. Reactions are presented, discussed, and illustrated in each of these items through various representative examples. The mechanistic hypotheses explaining the origin of the regioselectivity in the hydration reactions are discussed. This review of recent advances in the regioselective hydration of internal unsymmetrical alkynes (from 2007 until the present) aims to update the current body of knowledge and aid researchers working in this field.

The degradation of chlorine-containing volatile organic compounds (Cl–VOCs) utilizing catalytic combustion technology is subject to the paradox of toxic byproduct formation and catalyst chlorine poisoning. Herein, a CaO-assisted strategy is proposed to resolve the awkward stuff for improving the catalytic combustion performance of chlorobenzene on SmMn2O5. The CaO-collaborated SmMn2O5 exhibits a significant decrease in T90 by 142 and 125 °C compared with unmodified and inert SiO2-composited SmMn2O5, respectively. The integrated characterization results confirm that CaO collaboration causes electron transfer from CaO to SmMn2O5, resulting in a reduction of the orbital overlap between Mn and O atoms to activate lattice oxygen (Olatt). The activated Olatt enables chlorobenzene to combust completely at a lower temperature of 275 °C at which toxic byproducts are not generated. Furthermore, the switching of the dechlorination site from SmMn2O5 to CaO avoids chlorine poisoning of active sites on SmMn2O5 and thus endows CaO-collaborated SmMn2O5 with prominent stability.

Solar hydrogen production by metal-free photocatalysts represents one of the important routes to realize a low-carbon energy system. Herein, the core–shell β-silicon carbide@potassium-doped polymeric carbon nitride (β-SiC@PPCN) heterojunction with β-SiC as a core and PPCN as a shell for photo-thermo catalytic hydrogen production is developed. With such a heterojunction, not only can the H–OH bond of absorbed water be activated, but also the migration of photogenerated carriers can be promoted due to the created internal-electric-field. Owing to the excellent photothermal conversion property of β-SiC, the temperature-dependent catalytic activity is also studied. The core–shell β-SiC@PPCN heterojunction can be run at an elevated temperature upon illumination, which enhances photoinduced electron–hole separation. Density functional theory calculations demonstrate that the elevated running temperature can activate absorbed water. Remarkably, the synergy of photocatalysis and thermocatalysis makes the core–shell β-SiC-50@PPCN heterojunction yield a photo-thermo catalytic hydrogen production rate as high as 13046.7 μmol·g–1·h–1. The present study provides a promising strategy for large-scale solar hydrogen production.

Using first-principles calculations, we showed that the monoclinic WO3(001) preferentially forms a reconstructed monolayer on the anatase TiO2(001) surface. We thoroughly examined the structure of the WO3/TiO2 surface under ambient conditions, i.e., in equilibrium with gas-phase O2/H2O or H2/H2O under a range of pressure and temperature or in aqueous solution under a range of pH and electrochemical potential. Based on the WO3/TiO2 surface structures at different potentials, we proposed the proton-coupled electron-transfer (PCET) reaction pathway during charging and oxygen reduction reaction (ORR) pathways during discharging, which account for its reversible electron storage ability. With electronic structure analysis, we depicted the charge separation effect of WO3 on TiO2 and the electron storage effect of WO3.

The coupling reaction of CO2 and n-butane was conducted over different metal-modified (Mn, Zr, Ni, Ti, Zn) ZSM-5 catalysts. A high CO2 conversion of 26.5% and n-butane conversion of 100% with the aromatics selectivity of 69.1% was achieved at a CO2 to n-butane ratio of 0.95 over Zn/ZSM-5. CO2 addition promoted BTX & olefin selectivity, while it inhibited alkane & A9+ formation. A detailed analysis showed that 13% of the carbon atoms from CO2 were incorporated in the generation of aromatic hydrocarbons. Oxygenated intermediates, such as aliphatic alcohol, aliphatic ketones, and substituted cyclopentenones, were detected sequentially with the increase of the reaction temperature. In addition, reverse Boudouard reaction, water gas shift reaction, and dry reforming also participated in the formation of CO over Zn/ZSM-5. Based on these findings and the detailed characterization results, a plausible mechanism of direct and indirect incorporation of carbon from CO2 into aromatics was proposed for the coupling reaction.

Tetrasubstituted alkenes are one of the most important classes of aggregation-induced emission luminogens with wide applications in analytical chemistry, bioimaging science, luminescent materials and cancer treatment. However, general methods for the assembly of tetrasubstituted alkenes, especially for unsymmetrical species, remain elusive. The established methods typically require prefunctionalized substrates, sensitive organometallic reagents and multiple-step operation. Herein, we report a dual phosphine/rhodium catalysis for the direct synthesis of unsymmetrical tetrasubstituted alkenes from readily available cyclopropenones, aryl halides and water. This reaction was achieved in a highly efficient and chemoselective manner by a synergistic merger of phosphine-mediated hydration with rhodium-catalyzed arylation, furnishing a diverse set of carboxyl acid–based tetrasubstituted alkenes (>80 examples). The resulting carboxylate groups can be employed as universal tags for the downstream synthesis of unsymmetrical tetraarylethenes. A range of the obtained compounds were demonstrated to be solid-state luminogens with obvious aggregation-induced emission properties. The success of late-stage functionalization of bioactive compounds further illustrate the synthetical utility of this protocol in material development and drug discovery.

Understanding the reduction mechanism of ZnO/CuOx interfaces by hydrogen is of great importance in advancing the performance of industrial catalysts used for CO and CO2 hydrogenation to oxygenates, the water-gas shift, and the reforming of methanol. Here, the reduction of pristine and ZnO-modified CuOx/Cu(111) by H2 was investigated using ambient-pressure scanning tunneling microscopy (AP-STM), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and density functional theory (DFT). The morphological changes and reaction rates seen for the reduction of CuOx/Cu(111) and ZnO/CuOx/Cu(111) are very different. On CuOx/Cu(111), perfect “44” and “29” structures displayed a very low reactivity toward H2 at room temperature. A long induction period associated with an autocatalytic process was observed to enable the reduction by the removal of chemisorbed nonlattice oxygen initially and lattice oxygen sequentially at the CuOx–Cu interface, which led to the formation of oxygen-deficient “5–7” hex and honeycomb structures. In the final stages of the reduction process, regions of residual oxygen species and metallic Cu were seen. The addition of ZnO particles to CuOx/Cu(111) opened additional reaction channels. On the ZnO sites, the dissociation of H2 was fast and H adatoms easily migrated to adjacent regions of copper oxide. This hydrogen spillover substantially enhanced the rate of oxygen removal, resulting in the rapid reduction of the copper oxide located in the periphery of the zinc oxide islands with no signs of the reduction of ZnO. The deposited ZnO completely modified the dynamics for H2 dissociation and hydrogen migration, providing an excellent source for CO2 hydrogenation processes on the inverse oxide/metal system.

The catalytic conversion of greenhouse gas CO2 into valuable chemicals is a vital goal toward carbon balance and sustainability. In recent decades, the chemical fixation of CO2 into cyclic carbonates has gained much attention. In this work, a series of zinc complexes bearing tetradentate aminopyridine (N4) ligands have been synthesized and characterized. These zinc complexes were applied to the coupling of CO2 with epoxides in excellent yields and with a broad substrate scope under cocatalyst- and solvent-free conditions. Moreover, the zinc catalysts could be readily recovered and reused five times without an obvious loss in catalytic activity. Based on spectroscopic characterizations and experimental results, catalyst Zn-3 (DAP-ZnBr2, DAP = 1,4-bis(2-pyridymethyl)-1,4-diazepane) has been found to be a multifunctional catalyst because of the presence of a Lewis acidic zinc center and a nuclephilic halide anion, and one pyridine is released for the activation of CO2 during the reaction.

Enzyme discovery and directed evolution are the two major contemporary approaches for the improvement of industrial processes by biocatalysis in various fields. Customization of catalysts for improvement of single enzyme reactions or de novo reaction development is often complex and tedious. The success of screening campaigns relies on the fraction of sequence space that can be sampled, whether for evolving a particular enzyme or screening metagenomes. Ultrahigh-throughput screening (uHTS) based on in vitro compartmentalization in water-in-oil emulsion of picoliter droplets generated in microfluidic systems allows screening rates >1 kHz (or >107 per day). Screening for carbohydrate-active enzymes (CAZymes) catalyzing biotechnologically valuable reactions in this format presents an additional challenge because the released carbohydrates are difficult to monitor in high throughput. Activated substrates with large optically active hydrophobic leaving groups provide a generic optical readout, but the molecular recognition properties of sugars will be altered by the incorporation of such fluoro- or chromophores and their typically higher reactivity, as leaving groups with lowered pKa values compared to native substrates make the observation of promiscuous reactions more likely. To overcome these issues, we designed microdroplet assays in which optically inactive carbohydrate products are made visible by specific cascades: the primary reaction of an unlabeled substrate leads to an optical signal downstream. Successfully implementing such assays at the picoliter droplet scale allowed us to detect glucose, xylose, glucuronic acid, and arabinose as final products of complex oligosaccharide degradation by glycoside hydrolases by absorbance measurements. Enabling the use of uHTS for screening CAZyme reactions that have been thus far elusive will chart a route toward faster and easier development of specific and efficient biocatalysts for biovalorization, directing enzyme discovery by challenging catalysts for reaction with natural rather than model substrates.

Hydrogen is an attractive energy carrier because of its high energy density and clean emission. Herein, we report the construction of Ptδ+–O(H)–Ti3+ species on titania-supported Pt nanoparticles with strong metal–support interaction (SMSI), which boost the catalytic production of hydrogen from methanol steam reforming and water-gas shift at low temperatures. Characterizations of in-situ FTIR spectroscopy confirmed the formation of Ptδ+–O(H)–Ti3+ species, which resulted from the reduction of titania and dissociation of water at the Pt–titania interfaces. Compared with the general titania-supported Pt NPs without SMSI, the Pt/TiO2 with SMSI exhibited an improved hydrogen production rate by 9 times, and the CO concentration in the effluent was lower than 200 ppm. These findings gave a model for exploring the structure-performance interplay and provided an efficient strategy to optimize the catalysts to accelerate the production of hydrogen.

Photocatalytic production of H2O2 from earth-abundant water and oxygen using low-cost metal-free carbon nitrides (CNs) through oxygen reduction is a prospective route toward a greener future. However, the H2O2 productivity is restricted by rapid electron–hole separation and the low oxygen reduction activity of CNs. Herein, we rationally designed a series of CNs with covalently bonded dual-functional ligands acting as electron acceptors and active sites to achieve high photocatalytic H2O2 production and superior stability. The best-performing carbon nitride displays a H2O2 production rate of 7.3 mmol/g h with an apparent quantum efficiency of 20.2% at 420 nm using formic acid as the electron donor. Moreover, the modified CNs show excellent stable H2O2 generation over 110 h without significant decline. Mechanistic studies reveal that H2O2 was produced through a 2e– oxygen reduction reaction route. Photoluminescence, photo-electrochemical, and Kelvin probe force microscopy results together with theoretical calculations have revealed that the excellent photocatalytic performance originates from the dual-functional ligand. It not only acts as an electron acceptor to promote photogenerated charge carrier separation by withdrawing electrons but also works as an active site to accelerate oxygen reduction by lowering the oxygen adsorption and activation energy. Moreover, this facial strategy of grafting ligands provides a universal approach to synthesize photocatalysts with enhanced reactivity under mild conditions by choosing the proper ligands for a specific reaction.

The role of the counterion in metal-catalyzed reactions can be crucial as this often-unconsidered component of the catalyst can modify the performance of the catalyst and influence the reaction rate and/or selectivity of the transformation under study. Herein, we disclose the effects of counterion variation in cationic halogen bond-assembled Rh(I) catalysts in the hydroboration reaction of terminal alkynes, which leads to rather elusive branched (or internal) hydroboration products. Our studies showed that the higher the coordination ability of the counterion, the higher the activity and selectivity toward the hydroboration products. This observation was demonstrated by catalytic and spectroscopic (NMR, IR and X-ray) studies. An array of structurally diverse alkynes was efficiently transformed into the corresponding hydroboration products employing the highest performing catalyst XBphos-Rh-OTf. The practicality of our synthetic method was demonstrated by developing one-pot hydroboration/Csp2-Csp2 coupling processes.

[FeFe] hydrogenases, metalloenzymes catalyzing proton/dihydrogen interconversion, have attracted intense attention due to their remarkable catalytic properties and (bio-)technological potential for a future hydrogen economy. In order to unravel the factors enabling their efficient catalysis, both their unique organometallic cofactors and protein structural features, i.e., “outer-coordination sphere” effects have been intensively studied. These structurally diverse enzymes are divided into distinct phylogenetic groups, denoted as Group A–D. Prototypical Group A hydrogenases display high turnover rates (104–105 s–1). Conversely, the sole characterized Group D representative, Thermoanaerobacter mathranii HydS (TamHydS), shows relatively low catalytic activity (specific activity 10–1 μmol H2 mg–1 min–1) and has been proposed to serve a H2-sensory function. The various groups of [FeFe] hydrogenase share the same catalytic cofactor, the H-cluster, and the structural factors causing the diverging reactivities of Group A and D remain to be elucidated. In the case of the highly active Group A enzymes, a well-defined proton transfer pathway (PTP) has been identified, which shuttles H+ between the enzyme surface and the active site. In Group D hydrogenases, this conserved pathway is absent. Here, we report on the identification of highly conserved amino acid residues in Group D hydrogenases that constitute a possible alternative PTP. We varied two proposed key amino acid residues of this pathway (E252 and E289, TamHydS numbering) via site-directed mutagenesis and analyzed the resulting variants via biochemical and spectroscopic methods. All variants displayed significantly decreased H2-evolution and -oxidation activities. Additionally, the variants showed two redox states that were not characterized previously. These findings provide initial evidence that these amino acid residues are central to the putative PTP of Group D [FeFe] hydrogenase. Since the identified residues are highly conserved in Group D exclusively, our results support the notion that the PTP is not universal for different phylogenetic groups in [FeFe] hydrogenases.

A highly chemo- and regioselective decarboxylative Heck-type coupling of carboxylic acids and terminal olefins has been developed using a catalytic system composed of Pd(OAc)2 in the presence of phosphine-sulfonamido ligands. Using the bulky ligand L1 leads to high selectivity for 1,1-disubstituted (branched; b) olefins that are generally difficult to obtain. The influence of all relevant reaction parameters was evaluated using a combination of design of experiments and one factor at a time optimization. The coupling of dimethoxy-benzoic acid with various olefinic substrates gave the corresponding branched olefins with excellent regioselectivity (b/l up to 42:1) in up to 80% isolated yields. In contrast, using the less bulky ligand L2 results in the inverse regioselectivity leading to the 1,2-disubstituted (linear; l) product again in high yield of 86% (b/l = 1:26). Detailed investigation of the mechanistic pathways by DFT calculations reveals that the sterically demanding aryl substituent at the sulfonamide group of ligand L1 favors the pathway via 1,2-insertion with an energy preference of 4.4 kcal/mol, thus furnishing the 1,1-disubstituted branched olefins. In contrast, the 2,1-insertion reaction is advantageous by 1.3 kcal/mol for the analogous less bulky ligand L2 leading to the linear products.

Hydrogenation of toluene (TOL) to methylcyclohexane (MCH) is one of the hydrogen carrier systems desired for social integration. Supported Pt nanoparticle catalysts are effective for this application. However, Pt is rare, expensive, and in short supply, limiting its practical applications. Therefore, the key issue for TOL hydrogenation is how to substantially reduce the amount of Pt required for the catalyst. Because a specific ensemble of Pt atoms, that is dominantly formed on the surface of the Pt nanoparticle, is required for achieving higher catalytic performance, there is a limit to the number of precious Pt that can be conserved by simply reducing the particle size. The structure sensitivity established in the existing heterogeneous catalyst so far makes it difficult to design precious metal-conserving catalysts with both high activity and atomic efficiency. Here, a strategy for breaking the above limitations is reported. Our approach uses the heteroatom ensemble (HAE) on Pt single-atom alloyed 3d transition-metal nanoparticle catalysts (Pt1M SAAs, M = Co, Ni, Cu). The role of the TOL fixation/activation site is assigned to the atomic M sites on HAE, whereas the H2-activation site is to the Pt single-atom site on HAE. The atomic-scale division of roles within the HAE improves the efficiency of competitive adsorption of TOL/H2, which is important for boosting TOL hydrogenation. To maximize the synergistic effect at the adjacent sites, the atomic composition, geometric configuration, and electronic state of these active sites as well as the density of the HAE were tuned by the chemical composition and particle size of Pt1M SAAs. High activity was observed on the Pt1Co SAA with a particle size of 1.8 nm and Pt/Co molar ratio of 0.002. The Pt mass-specific activity reached 219 mol/gPt/h, which was 23 times higher than that in a conventional Pt nanoparticle-supported catalyst. Using a set of well-defined Pt1M SAAs, high-angle annular dark-field scanning transmission electron microscopy, Pt LIII-edge X-ray absorption fine structure spectroscopy, coupled with periodic density functional theory and ab initio molecular dynamics simulation, we proved the origin of the structure sensitivity at an atom-to-nanometer scale. The present work sheds light on the significance of regulations of the coordination environment of the Pt single-atom site, atomic composition, and particle size of Pt1M SAA for creating high activity, durability, and Pt-utilization efficiency for catalytic applications relevant to hydrogen carrier systems.